1. Field of the Invention
The present invention relates to a projection system incorporating a reflection-type optical element, such as a DMD (Digital Micromirror Device) for example, and an optical element such as a color filter. The invention also relates to a method of operating such a projection system.
2. Description of the Related Art
A common optical element 4 called DMD for use in this type of projection system, particularly home theater or RPTV (Rear Projection Television), includes a six-segment color filter (also called color wheel) 3 having six segments of the three primary colors arranged circumferentially in the sequence of R (red)-G (green)-B (blue)-R-G-B at about 60° intervals, as shown in FIG. 4. Recently, there has been developed a seven-segment color filter including a green ND color filter segment having a relatively low lightness in addition to the above-mentioned six segments, as shown in FIG. 5. The “ND”, as used herein, is an acronym for Neutral Density, which means a relatively low lightness.
Reflected light Φ3 from the optical element 4 is imparted with gradation by turning ON/OFF each of micromirrors included in the optical element 4. A halftone of a color is produced by adjustment of the duration of ON state. The six-segment type optical element usually provides gradation based on 8-bit resolving power, whereas the seven-segment type optical element displays each micromirror by, for example, 10-bit driving when a bundle of rays Φ2 having passed through ND filter segment 3g is reflected by the micromirror, thereby improving the reproducibility of a dark portion of an image. As a result, the seven-segment type optical element can realize superior gradation throughout all the levels of gray.
One example of a projection system using such a seven-segment type optical element is HOME THEATER XV-Z1000 manufactured by SHARP CORPORATION. A typical discharge lamp 1 for use in this projection system is adapted to d.c. lighting with d.c. lamp current (Io). The aforementioned ND filter is relatively low in lightness and hence has a low light transmittance. Accordingly, with the discharge lamp 1 driven with lamp current Io, the seven-segment type projection system lessens the luminous flux of light Φ2 having passed through the color filter 3 as compared to the six-segment type projection system, thus resulting in a problem of a lowered screen illuminance. If the ND color filter segment 3g occupies a 30° area, the screen illuminance is lowered by 15-20%. This is because, though the proportion of the area of the ND color filter segment 3g to the entire area of the color filter 3 is 8% (30°/360°=0.08), the ND color filter segment 3g, which is a green area having a high luminous efficiency, contributes to such a large decrease in illuminance.
FIG. 3 shows the waveform of d.c. lamp current driving a conventional d.c. lamp and the waveform of the luminous flux of light Φ2 having passed through the color filter 3. Here, a simple d.c. lighting current F is plotted with a solid line, while a current resulting from superimposition of pulse current IR on the simple current at color filter R (red) for arc stabilization is plotted with a broken line. In FIG. 3, a mean lamp current Io is equal to the simple d.c. lighting current F.
As shown in FIG. 3(iii), the luminous flux of light Φ2 having passed through the color filter 3 varies in accordance with color segments 3a to 3g. Specifically, the luminous flux of light Φ2 having passed through a G (green) color segment having a relatively high luminous efficiency is relatively high, whereas the luminous flux of light Φ2 having passed through a B (blue) color segment having a relatively low luminous efficiency is relatively low. The luminous flux of light Φ2 having passed through an R (red) color segment assumes a value around the midpoint between that of light having passed through the G (green) color segment and that of light having passed through the B (blue) color segment. The luminous flux of light Φ2 having passed through an ND (dark green) color segment assumes the lowest value. The mean luminous flux of light Φ2 is represented by Φ2a in the figure. The ND color filter segment 3g lowers the illuminance of light Φ2 having passed through the color filter 3 largely at mean lamp current Io, which sacrifices the mean luminous flux Φ2a thereby lowering the illuminance of light Φ2 as a while.
Accordingly, it is an object of the present invention to provide a projection system which is configured to lower the value of d.c. lamp current only during an ND filter area illuminating period in terms of the time axis of d.c. lamp current thereby providing the advantages of: (1) further improving the effect of the ND filter on a resulting image; (2) minimizing the decrease in screen illuminance notwithstanding the presence of the ND filter; and (3) improving arc stability.
Another object of the present invention is to provide a method of operating such a projection system.